As sizes and weights of portable electronic devices have been reduced, high-performance and high-capacity batteries for power sources thereof have been increasingly demanded.
Lithium ion rechargeable batteries have been widely used as power sources of portable devices since 1991 because of their small, light, and high-capacity characteristics. Recently, electronic, information and communication devices (e.g., camcorders, cell phones, and laptop computers) have been markedly developed with the rapid development of electronic, communication, and computer industries, so the lithium ion rechargeable batteries have been increasingly demanded to be used as power sources of these devices.
In particular, a power source for a hybrid electric vehicle having both an internal combustion engine and the lithium ion rechargeable battery is being actively researched in the United States, Japan, Europe, and so on. The lithium ion rechargeable battery is considered as a large battery for an electric vehicle because of its energy density, but it is in an early stage of the development and has a stability problem. A nickel metal hydride battery has been used in the electric vehicle because of its stability. In other words, the lithium ion rechargeable battery has high-cost and stability problems.
In particular, commercially available cathode active materials (e.g., LiCoO2 and LiNiO2) have an unstable crystal structure due to delithiation upon charging the battery, so thermal characteristics of the cathode active materials are very poor. In other words, if the overcharged battery is heated at a temperature of 200° C. to 270° C., the structure of the battery is rapidly changed to cause an oxygen emitting reaction in lattices of the changed structure (J. R. Dahn et al., Solid State Ionics, 69, 265(1994)).
To solve this problem, it has been attempted to substitute a portion of nickel with a transition metal element to increase an exothermic start temperature or broaden an exothermic peak preventing a rapid exothermic reaction (T. Ohzuku et al., J. Electrochem. Soc., 142, 4033 (1995), Japanese Patent laid-open Publication No. 1997-237631). However, the results have not yet been satisfied.
In addition, LiNi1-xCoxO2 (x=0.1-0.3) materials in which a portion of nickel is substituted with thermally stable cobalt shows good charge-discharge and lifetime characteristics, but it cannot provide thermal stability.
Furthermore, a Li—Ni—Mn-based composite oxide and a Li—Ni—Mn—Co-based composite oxide and methods of preparing the same have been suggested. The Li—Ni—Mn-based composite oxide is prepared by substituting a portion of nickel (Ni) with thermally stable manganese (Mn), and the Li—Ni—Mn—Co-based composite oxide is prepared by substituting a portion of Ni with Mn and cobalt (Co). For example, Japanese Patent laid-open Publication No. 1996-171910 discloses a method of preparing a cathode active material of LiNixMn1-xO2 (0.7≦x≦0.95), and the method includes: mixing an aqueous solution of Mn and Ni with an alkaline solution to co-precipitate Mn and Ni; mixing the co-precipitated compound with lithium hydroxide; and firing the mixture of the co-precipitated compound and the lithium hydroxide. Japanese Patent laid-open Publication No. 2000-227858 discloses a cathode active material in which a compound of Mn and Ni compounds is uniformly distributed at an atomic level to provide a solid solution instead of the concept that the transition metal element is partially substituted into LiNiO2 or LiMnO2.
According to European Patent No. 0918041 or U.S. Pat. No. 6,040,090, LiNi1-xCoxMnyO2 (0<y≦0.3) has improved thermal stability compared to that of materials composed of only Ni and Co. However, LiNi1-xCoxMnyO2 (0<y≦0.3) may not be commercially developed due to its reactivity with an electrolytic solution of Ni4+. In addition, European Patent No. 0872450 discloses LiaCobMncMdNi1-(b+c+d)O2 (M=B, Al, Si, Fe, Cr, Cu, Zn, W, Ti, or Ga) in which Ni is substituted with another metal as well as Co and Mn. However, the thermal stability of the Ni-based material is not yet solved.
To solve this problem, Korean Patent Laid-open Publication No. 2006-00355547 discloses a double-layered cathode active material including an internal part composed of a nickel-based internal active material and an external part composed of a transition-metal composite active material. The nickel-based internal active material has a high capacity characteristic, and the transition-metal composite active material has a high stability characteristic. Thus, the double-layered cathode active material has the high capacity characteristic, a high charging density, an improved lifetime characteristic, and excellent thermal stability.
In addition, Korean Patent No. 10-0744759 discloses a cathode active material in which a metal composition is distributed in a continuous concentration gradient from an interface between a core and an outer shell to a surface of the active material. The cathode active material disclosed in Korean Patent No. 10-0744759 has excellent thermal stability.
To prepare the cathode active material, a heat treatment may be performed after preparing a precursor. However, since a metal is diffused during the heat treatment, it is difficult to prepare the outer shell having the transition metal of a desired concentration.